1. Field of the Invention
The present invention relates to a device and method for reaction injection molding. More specifically, the present invention relates to device and method for reaction injection molding while allowing for the use of retractable pins. Still more specifically, the present invention relates to a device and method for forming a layer on a golf ball using retractable pins by reaction injection molding.
2. Description of the Related Art
The covers of today's golf balls are made from a variety of materials, such as balata, SURLYN®, and IOTEK®. Balata is a natural or synthetic trans-polyisoprene rubber. Balata covered balls are favored by more highly skilled golfers because the softness of the cover allows the player to achieve spin rates sufficient to more precisely control ball direction and distance, particularly on shorter shots. Balata-covered balls, however, are easily damaged, and thus lack the durability required by the average golfer. Accordingly, alternative cover compositions have been developed in an attempt to provide balls with spin rates and a feel approaching those of balata-covered balls, while also providing higher durability and overall distance.
Ionomer resins have, to a large extent, replaced balata as a cover material. Chemically, ionomer resins are a copolymer of an olefin and an α, β-ethylenically-unsaturated carboxylic acid having 10 to 90 percent of the carboxylic acid groups neutralized by a metal ion, as disclosed in U.S. Pat. No. 3,264,272. Commercially available ionomer resins include, for example, copolymers of ethylene and methacrylic or acrylic acid, neutralized with metal salts. Examples of commercially available ionomer resins include, but are not limited to, SURLYN® from DuPont de Nemours and Company, and ESCOR® and IOTEK® from Exxon Corporation. These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization. However, while ionomer-covered golf balls possess virtually cut-proof covers, the spin and feel are inferior compared to balata-covered balls.
Polyurethanes have also been recognized as useful materials for golf ball covers since about 1960. The resulting golf balls are durable and, unlike ionomer-covered golf balls, polyurethane golf ball covers can be formulated to possess the soft “feel” of balata-covered golf balls. U.S. Pat. No. 4,123,061 teaches a golf ball made from a polyurethane prepolymer formed of polyether with diisocyanate that is cured with either a polyol or an amine-type curing agent. U.S. Pat. No. 5,334,673 discloses the use of two categories of polyurethane available on the market, i.e., thermoset and thermoplastic polyurethanes, for forming golf ball covers and, in particular, thermoset polyurethane-covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent, and/or a difunctional glycol.
Polyureas have also been proposed as cover materials for golf balls. For instance, U.S. Pat. No. 5,484,870 discloses a polyurea composition comprising the reaction product of an organic diisocyanate and an organic amine, each having at least two functional groups. Once these two ingredients are combined, the polyurea is formed, and thus the ability to vary the physical properties of the composition is limited.
Conventionally, golf balls are made by molding a cover around a core. The core may be wound or solid. A wound core typically comprises elastic thread wound about a solid or liquid center. Solid cores typically comprise a single solid piece center or a solid center covered by one or more mantle or boundary layers of material. Wound cores may also include one or more mantle layers.
The cover may be injection molded, compression molded, or cast over the core. Injection molding typically requires a mold having at least one pair of mold cavities; e.g., a first mold cavity and a second mold cavity, which mate to form a spherical recess. In addition, a mold may include more than one mold cavity pair.
In one exemplary injection molding process, each mold cavity may also include retractable positioning pins to hold the core in the spherical center of the mold cavity pair. Once the core is positioned in the first mold cavity, the respective second mold cavity is mated to the first to close the mold. A cover material is then injected into the closed mold. The positioning pins are retracted while the cover material is flowable to allow the material to fill in any holes caused by the pins. When the material is at least partially cured, the covered core is removed from the mold (demolded).
Compression molds also typically include multiple pairs of mold cavities, each pair comprising first and second mold cavities that mate to form a spherical recess. In one exemplary compression molding process, a cover material is pre-formed into half-shells, which are placed, respectively, into each of a pair of compression mold cavities. The core is placed between the cover material half-shells and the mold is closed. The core and cover combination is then exposed to heat and pressure, which cause the cover half-shells to combine and form a full cover.
Casting processes also typically utilize pairs of mold cavities. In a casting process, a cover material is introduced into a first mold cavity of each pair. A core is then either placed directly into the cover material or is held in position (e.g., by an overhanging vacuum or suction apparatus) to contact the cover material in what will be the spherical center of the mold cavity pair. Once the cover material is at least partially cured (e.g., to a point where the core will not substantially move), the cover material is introduced into a second mold cavity of each pair, and the mold is closed. The closed mold is then subjected to heat and pressure to cure the cover material thereby forming a cover on the core.
As a common feature of injection molding, compression molding, and casting, when used to form a golf ball cover, the mold cavities typically include a negative dimple pattern to impart a dimple pattern on the cover during the molding process.
Casting is the most common method of producing a urethane or urea layer on a golf ball. However, the materials typically used in casting require a relatively long gel time. Long gel times have the disadvantage of requiring long cure times for the material to set so that the ball can be demolded, or removed from the mold. Additionally, once demolded, cast golf balls usually require subsequent buffing and other finishing process steps. Another disadvantage of using materials with a long gel time is that they may require sacrificing one or more material properties, such as flex modulus or resiliency.
Reaction Injection Molding (RIM) allows for a wider range of materials to be used in manufacturing, including materials with a short gel time. RIM, however, is subject to technical challenges, one of which is eliminating or minimizing the production of flash. Flash is extra material formed during molding or casting that must subsequently be removed. Since the materials used in RIM can have low viscosity, they readily flow into any crevices or holes within the mold. If retractable pins are used, there will necessarily be some clearance between the pins and the holes in the mold in which the pins are mounted. Thus, low viscosity layer-forming materials have not heretofore been usable with retractable pin reaction injection molding. As a result, conventional RIM has been limited to using materials having longer gel times. Otherwise, extensive and oftentimes economically prohibitive post-mold processing is required to remove the resulting flash. Furthermore, extensive labor is often required to clean and maintain the mold after retractable pin reaction injection molding.
What is needed is an improved retractable pin reaction injection molding device and method allowing for injection of materials with low viscosity.